AGS alignments in lunar orbit and operations on the lunar surface

See list attachedApril 4, 196969-PA-T-55APA/Chief, Apollo Data Priority CoordinationAGS alignments in lunar orbit and operations on the lunar surface

On April 2 we finally got around to establishing how to operate the AGS on the lunar landing mission. The two basic subjects for dis- cussion were how to handle CDU transient problems when aligning the AGS to the PGNCS in lunar orbit and how to operate the AGS in total while on the lunar surface.

I am certainly no authority on CDU transients and only attempt the following brief description so that the rest of the memo will make some sense to you. If you are interested in what CDU transients really are, I recommend that you find an authority on them. There are lots of 'em – and as many versions. As you know, the AGS uses the PGNCS as the primary reference in its alignments. As I under- stand it, CDU transients have something bad to do with the electronics in the PGNCS which are used to generate the data transmitted to the AGS which the AGS uses in its alignments. Unless certain precautions are taken, CDU transients can occur and are not ordinarily obvious to the crew. I gather that they can result in errors in the AGS alignments of up to 1½ degrees or so. During much of the operation even the largest misalignment errors would not particularly concern us. On other occasions, such as during descent, they would essentially disable the AGS as a useful guidance and control system.

I will go through each of the AGS alignments:

a. LM Activation before Undocking

The command module should be used to orient the spacecraft to a so-called AGS calibration attitude which is essentially just displacing all three spacecraft axes at least 11½ degrees away from zero or multiples of 45 degrees from the IMU principle axes. This action, it is said, will permit the AGS alignment and calibration to be carried out free of CDU transients.

b. Pre-DOI after Undocking

The AGS is aligned to the PGNCS after its AOT alignment in preparation of DOI. Since AGS alignment errors do not create a problem but are more of an annoyance in the AGE monitoring of the DOI burn, no precautions will be taken to avoid CDU transients.

c. Pre-PDI

This alignment in preparation for descent is most critical. The AGS must be aligned accurately and, in order to minimize drift, it must be aligned to the PGNCS very late before PDI. The choices here were to add special crew procedures into an already crowded timeline to avoid CDU transients vs. taking no precautions against their occur- ring, but being prepared to redo the alignment if the MCC detects a CDU transient alignment error has occurred. Either of these two approaches were considered acceptable and are almost a toss-up. It was finally decided to avoid the special procedures and to take a chance on the transient. If the MCC determines that a CDU transient has occurred, the crew will be informed within 30 seconds and they must then rezero the CDU's and repeat the alignment. This procedure is felt to be simpler for the crew and, in particular, it avoids attitude maneuvers which are part of the CDU transient avoidance procedure.

d. Post-Insertion Alignments

After insertion into orbit the AGS should then be aligned to the PGNCS. Again in this non-critical period it was decided to take a chance on a CDU transient occurring, particularly since this align- ment is carried out within sight of the earth and the MCC is in a position to advise the crew if a realignment is necessary.

Attached to this memo is a detailed sequential list of AGS options on the lunar surface at each step of which it is assumed the PGNCS is still operational. In other words, it is the nominal sequence. If the PGNCS becomes broken on the lunar surface, different and more extensive operations will be required, which we have yet to define. In the development of the attached sequences, some items of interest and action items popped out which I would like to add here.

a. Whenever RLS is updated in the PGNCS, it should be standard procedure to update the AGS lunar launch site radius (Address 231). This update will be based on a voice relay from the MCC of the value to be input via the AGS DEDA by the crew.

b. With regard to CDU transients during AGS alignments on the lunar surface, it was decided that we would rely on the MCC to monitor and advise the crew if a CDU transient has occurred. That is, the crew would follow no special procedure to determine if one had occurred except in the case of no communication.

c. Guidance and Control Division and TRW were requested to advise what timetag should be associated with the CSM state vector voiced to the crew for input into the AGS in the event the PGNCS has failed.

d. MPAD was asked to determine if it is acceptable to input state vectors into the AGS 15 minutes or more prior to PDI. The question here really is whether or not the AGS numerical integration causes unacceptable state vector errors for descent aborts if the state vectors are loaded too early. Early loading, of course, is desirable to reduce crew activity just before PDI.

All of this AGS jazz will be added to the Lunar Surface Mission Techniques Document. I think it's the last chunk. We will review the whole subject of lunar surface activity next week and then can forget it – I hope.

Enclosure 1

April 2, 1969

First Two Hours on the Lunar Surface After Touchdown and First Stay Decision

1. PGNCS goes to P68

2. 413 + 10,000 Lunar Surface flag to store azimuth and terminate average g

3. 414 + 10,000 State vector update (V47) after verification of PGNCS

4. 400 + 30,000 AGS align to PGNCS

5. 400 + 10,000 Initialize for Ascent

6. 413 + 10,000 Store better azimuth

7. Stay for two hours decision

8. Crew readout to MCC addresses 047 and 053

9. 400 + 60,000 AGS gyro calibration [5 minutes required]